The invention relates to an electron image projector wherein in operation electrons emitted by a cathode in a predetermined spatial pattern extending transverse to the direction of motion of the electrons are accelerated away from the cathode by an electric field and are focussed by a magnetic field so as to impinge on a target in a manner reproducing said spatial pattern thereon, the projector comprising anode means for accelerating the electrons away from the cathode, said anode means incorporating an intermediate electrode for effecting a homogeneous electric field in a region betwen the cathode and the anode.
Such an electron image projector is known from, No. EP-A-136 752 used in the fabrication of integrated circuits by projecting a mask pattern onto a semiconductor slice with high resolution. With substantially unity magnification, all parts of the usable area of the slice may be exposed simultaneously. In a typical projector, the mask and the slice are aligned parallel to one another a short distance apart. The mask comprises a disc of a material (such as quartz) transparent to ultra-violet radiation. The disc supports a layer of opaque material (such as chromium) in the desired pattern and an overall layer of photo-emissive material (such as CsI), and forms the cathode of an electron-optical system, of which the slice forms the anode (target). A large potential difference (typically 20-50 Kv) is applied between the cathode and anode, a uniform magnetic field (typically 1 kG or more) is applied parallel to the electric field between cathode and anode, and the layer of photo-emissive material on the mask is illuminated with UV-radiation. Electrons are emitted from those portions of the photo-emissive layer not screened from the UV-radiation by the pattern of opaque material, and are accelerated by the electric field to impinge on the anode. Electrons which are emitted at an angle to the normal to the cathode describe a helix about a magnetic field line as they travel from cathode to anode, the number of revolutions described being almost independent of the angle to the normal, and the distance between the cathode and anode being selected (in dependence on the values of the electric and magnetic fields) so that the number of revolutions is substanitally an integer, typically unity. Electrons emitted from any particular point on the cathode over a range of angles about the normal are thereby focussed to impinge on substantially the same point on the anode, so that the pattern of the cathode as a whole is reproduced on the anode with unity magnification. The anode is said to be at a magnetic focus with respect to the cathode, and when the above-mentioned integral number of revolutions is unity, the anode is said to be at the first magnetic focus.
Such an electron image projector has the disadvantage that the target forms an essential functional part of the electron-optical system, and therefore affects the exact configuration of the electric field (which ideally would be homogeneous and normal to the mask and target) and hence the electron trajectories in its vicintiy. In the fabrication of the integrated circuits, which requires the successive exposure of different patterns onto the slice (the slice being coated each time with a suitable electron-sensitive resist), this means that the slice should either be flat, or be reproducibly non-planar so as to reproduce the same distorted electric field pattern over the slice, and must each time be accurately located in the same position so as to reproduce the same distorted electric field pattern over the slice, and must each time be accurately located in the same position so as to reproduce the same distorted electric field pattern at the step which generally occurs at the edge of the slice (where it is mounted in or on a chuck), so as thereby to obtain optimum registration of the successive patterns. To satisfy these requirements is time-consuming and inconvenient, particularly (with regard to planarity) because the electron image projection has a large depth of focus (typically 100 .mu.m) and does not require the slice to be flat for adequate resolution over the slice. It may be noted that the magnetic field can be made slightly diverging or converging to compensate for net overall expansion or contraction of the slice relative to the mask between successive exposures. The additional electrode has the shape of a grid and provides the advantage that the dependence of the electron trajectories on the shape and/or disposition of the target can be reduced. In operation the electric field resulting from a potential difference between the grid means and the target may be much less than the electric field resulting from a potential difference between the cathode and the target, and said dependence is much reduced. Suitably, there is in operation substantially no potential difference between the grid and the target: said dependence can thereby be substantially eliminated, and other equipment may readily be used adjacent said region without substantially affecting the electron trajectories, as will be mentioned again below.
The surface of the grid facing the cathode may be substantially at the magnetic focus closet to the cathode. By positioning the grid means substantially at a magnetic focus, localized distortion introduced by the grid into the projection of the spatial pattern from the cathode to the target can be optimally corrected by the magnetic field, and selecting the first magnetic focus, that is the focus closest to the cathode, enables the projector to be relatively compact. With such an arrangement, wherein in operation the electric field resulting from a potential difference between the grid and the target is much less than the electric field resulting from a potential difference between the cathode and the grid, or wherein in operation there is substantially no potential difference between the grid and the target, the spacing between the grid means and the target suitably is substantially twice the spacing between the cathode and the grid. The target will thereby be substantially at the second magnetic focus, since the substantially constant velocity of the electrons between the grid and the target will be twice the average velocity of the electrons between the cathode and the grid.